CIRCULATION IN THE CAPILLARIES. 83 



A red corpuscle occasionally becomes involved in the still layer, when it 

 moves slowly, turning over and over, or even remains stationary for a time, 



FIG. 35. Small artery and capillaries from the lung of a frog ; magnified 500 diameters (from a 

 photograph taken at the United States Army Medical Museum). 



until it is taken up again and carried along with the central current. A few 

 leucocytes are constantly seen in this layer. They move along slowly and 

 apparently have a tendency to adhere to the walls of the vessel. This is due 

 to the adhesive character of the surface of the white corpuscles as compared 

 with the red, which can easily be observed in examining a drop of blood 

 between glass surfaces, the red corpuscles moving about freely, while the 

 white corpuscles have a tendency to adhere to the glass. 



Great differences exist in the character of the flow of blood in the three 

 varieties of vessels which are under observation. In the arterioles, which 

 may be distinguished from the capillaries by their size and the presence of 

 the muscular and fibrous coats, the movement is distinctly remittent, even in 

 their most minute ramifications. The blood moves in them with much 

 greater rapidity than in either the capillaries or veins. They become smaller 

 as they branch, and carry the blood always in the direction of the capillaries. 

 The veins, which are relatively larger than the arteries, carry the blood 

 more slowly and in a continuous stream from the capillaries toward thQ 

 heart. In both the arteries and veins the current is frequently so rapid 



